1. Field of the Invention
The present invention relates to a branching device, and particularly relates to a branching device including a plurality of filter devices and inductors.
2. Description of the Related Art
Branching devices such as duplexers that separate transmission signals and reception signals by frequency using surface acoustic wave filters (SAW filters) or the like are widely used in communication devices such as cellular phones and smartphones.
This type of branching device uses a ladder filter, in which a plurality of surface acoustic wave resonators are connected in ladder form, in order to widen a pass band, which is a frequency band where the transmission signals and the reception signals are allowed to pass. Furthermore, a configuration in which a plurality of inductors are added to the filters is also used to achieve impedance matching with a transmission/reception antenna and adjust the pass band characteristics of each filter.
Japanese Unexamined Patent Application Publication No. 2009-21895 discloses a configuration of a shared antenna device including first to third filters that are electrically connected to an antenna and that have pass bands at mutually different frequencies, in which an inductor for impedance matching is provided between an antenna terminal and a ground potential.
Additionally, Japanese Unexamined Patent Application Publication No. 2007-174100 discloses a configuration of a duplexer that separates transmission signals and reception signals, in which an inductor for antenna matching and an inductor for adjusting the characteristics of a transmission filter (extension inductor) are formed as meandering lines within a laminated substrate. In the duplexer disclosed in Japanese Unexamined Patent Application Publication No. 2007-174100, isolation characteristics are improved by making magnetic fields produced by the inductors orthogonal to each other to reduce the influence of electromagnetic fields acting on each other.
In communication devices such as cellular phones, it is necessary to further reduce the size of the device and precisely separate signals having a plurality of different pass bands. As such, in a branching device having a plurality of filters as described above, it is necessary to improve attenuation characteristics for signals outside of the target pass bands of the filters and improve the isolation characteristics among the filters.